Functional Intersection Area

An intersection is defined by both its functional and physical areas. The functional area of an intersection extends both upstream and downstream from the physical intersection area and includes any auxiliary lanes and their associated channelization.

The functional area on the approach to an intersection, driveway or entrance consists of three basic elements:

• Perception-reaction distance
• Maneuver distance
• Queue-storage distance

The distance traveled during the perception-reaction time will depend upon vehicle speed, driver alertness, and driver familiarity with the location. Where there is a left-turn lane, the maneuver distance includes the length needed for both braking and lane changing. In the absence of turn lanes, it involves braking to a comfortable stop. The storage length should be sufficient to accommodate the longest queue expected most of the time.

Ideally, driveways and entrances should not be located within the functional area of an intersection or in the influence area of an adjacent driveway or entrance.